This invention is directed to electronic distance measuring or gauging and, more particularly, to eddy current loss proximity distance measuring gauges.
Eddy current loss (ECL) sensors are well known and widely used to sense whether or not a mental object (usually called a target) is within a predetermined distance of the coil of the tuned circuit of the ECL sensor. Such devices are used on aircraft, for example, to determine whether the doors of the aircraft are closed or open. Functionally, an ECL sensor comprises a high Q coil that forms part of a tuned (tank) circuit. The coil generates a magnetic field and forms the inductor portion of an oscillator circuit. The oscillator circuit includes just sufficient positive feedback to sustain oscillation in the absence of a metal target being in the proximity of the coil. When such a target is brought into the magnetic field generated by the coil, eddy currents are produced due to the target's conductivity. These eddy currents absorb energy from the magnetic field and, thereby, increase the losses of the ECL oscillator's tuned circuit. When the Q of the coil decreases below a certain determinable value, oscillation terminates. The change of distance between the coil and the target from maximum oscillation to termination of oscillation is very small. Because this distance is very small, it is difficult to use an ECL sensor directly to provide a measuring device. Moreover, the amplitude of oscillation over the small separation distance is nonlinear.
The prior art has proposed modifying ECL sensors in a manner such that they can be better utilized as distance measuring devices. Examples of such proposals are described in U.S. Pat. No. 3,609,580, issued to Francis T. Thompson et al on Sept. 28, 1971 for "Electrical Sensing Apparatus"; U.S. Pat. No. 3,619,805, issued to Stephen F. Bean on Nov. 9, 1971 for "Noncontacting Displacement Transducer Including an Oscillator with Cable-Connected Inductive Probe"; and, U.S. Pat. No. 3,747,011 issued to Robert Buck on July 17, 1973 for "Metal Detector Including Proximity-Responsive Oscillator with Feedback-Stabilized Gain". For various reasons, the devices proposed by these patents have not been as successful as desired. For example, the Thompson patent merely describes a circuit wherein the normal oscillation frequencies are shifted to a lower level, it does not provide a circuit which expands the target/coil separation distance in a linear manner and, thereby, provides operation over a greater range. While the Buck patent does disclose a technique for expanding the target/coil separation distance, it does so in a nonlinear manner. Thus, the output of the circuit proposed by Buck is nonlinearly related to a particular distance between the target and the coil. The Bean patent merely discloses a system wherein the frequency of operation is reduced in order to allow the overall system to be utilized in combination with a probe spaced from the electronic system by a greater distance than that allowed by prior art devices.
Thus, in general, the prior art proposals for utilizing ECL sensors as distance measuring devices either do not extend the target/coil separation distance over which measurements can be made, or they do so in a nonlinear manner. In addition, to these disadvantages, the prior art devices in general are not as temperature stable as desirable. Obviously, temperature stability is very important when a highly linear output is desired.
Therefore, it is an object of this invention to provide a new and improved ECL proximity distance measuring gauge.
It is a further object of this invention to provide an ECL proximity distance measuring gauge having an output that is linearly related to the distance being measured.
It is a still further object of this invention to provide a new and improved linear ECL proximity distance measuring gauge that is automatically temperature stabilized.